Relatives of five-year-old brain cancer patient Ashya King, who was taken from hospital without doctors' consent, have hit out at the treatment of his parents who are being held in Spain.
A judge ruled that the couple must be held for up to 72 hours while the court considers whether to grant Britain an extradition request, the National Court in Madrid said.
Ashya's parents Brett and Naghmeh told the judge they do not want to return to the UK.
The Crown Prosecution Service (CPS) said the case is under review.
A centre in the Czech Republic which offers proton therapy said it was willing to help Ashya if his doctors in Britain agree.
The family took him from Southampton General Hospital last Thursday and travelled to France with him and his six siblings before heading to the Costa del Sol in southern Spain.
Mr King, 51, and Mrs King, 45, were arrested on Saturday night in Velez-Malaga by Spanish police.
Ashya is being treated in a Spanish hospital and his grandmother and brother have criticised the way his parents are being treated.
Ashya's brother, Naveed, told Channel 4 News: "We're not allowed to go and see Ashya at all.
"There is police standing outside his hospital room. We are not allowed to go and see him. We have tried to call the hospital but they are not revealing any information at all to us.
"My mum was by his (Ashya's) side for the whole month that he was in hospital so for him to now suddenly not be with anyone of the family ... his health might actually deteriorate because he can't be entertained and be happy."
Ashya's grandmother Patricia King said she spoke to her elder grandson Danny for the first time this morning.
She said: "Danny said they have taken his parents to Madrid and are giving them a terrible run-around.
"The whole thing is a huge injustice. They are still not allowed to see Ashya, which is shocking. It's the worst thing of all. If this carries on, it could kill him."
According to reports, the couple travelled to Spain to sell a holiday home to obtain funds for proton beam therapy, which is not available through the NHS.
Their son, who is suffering from a stage four brain tumour, is being cared for at the Materno-Infantil hospital in Malaga.
Danny told the BBC: "We are very grateful that he is in such a good hospital. The only thing my parents are worried about is Ashya, I hope my parents get released as soon as possible."
British police defended their decision to request a European arrest warrant.
Assistant Chief Constable Chris Shead, of Hampshire Constabulary, said he was aware the police's approach had created a "significant amount of debate" but he would rather be criticised for "being proactive" than "potentially having to explain why a child has lost his life".
Simon Hayes, police and crime commissioner for Hampshire and the Isle of Wight, said: " Hampshire Constabulary's role, as in many other cases, was to safeguard the interests of a very vulnerable sick young child and find Ashya."
The Prime Minister's official spokesman, asked whether David Cameron sympathised with Ashya's parents, said: "I think people up and down the country will understand and be moved by the grave illness from which Ashya is suffering.
"First and foremost, the priority must be that he receives the very best and most appropriate medical care. Of course, I am sure that every parent wants to do the best for their child. That is probably the most human of human instincts."
The CPS said the case was under "immediate review" and a decision will be made whether to prosecute.
A spokesman said it had applied for the arrest warrant "at the request of Hampshire Police for an offence of cruelty to a person under the age of 16 years" on Friday.
The CPS reviewed the evidence available at that time. "Further evidence is now being provided to the CPS and so the case is under immediate review."
The Proton Therapy Centre (PTC) in Prague confirmed it was able to treat Ashya immediately if he was eligible for therapy, with the cost of the treatment to be sorted out later.
Director of strategy Iva Tatounova said Naveed had been in touch with the centre today, giving details of the Spanish doctors treating Ashya, and saying they were willing to send the centre the result of his MRI scan.
"We have asked them to do that, so we can assess the boy's condition," she said.
"There would have to be a recommendation from the doctors in Southampton, but we are here, and willing to co-operate."
The hospital's medical director Dr Michael Marsh said he regretted its relationship with the Kings had broken down.
He said: "We were really concerned about Ashya's welfare when he went missing from our hospital last Friday and for a period of time we had no idea where he was or what his parents' intentions were.
"The chances of surviving the condition Ashya has are about 70-80% after five years so we believe that he has a good chance of a successful outcome provided he gets access to the most appropriate treatment.
"We have discussed proton beam radiotherapy with Ashya's family and there are some tumours that respond well to this type of treatment, but there are some cases where there isn't the evidence that this is a beneficial treatment. Where the evidence supports this treatment, we have made a referral and patients have been treated abroad."